Optical gas sensors operate with a light source (radiation source), a photosensor and a measuring section located between the light source and the photosensor. A wavelength range, in which the target gas has a characteristic absorption, can be selected with a band pass filter. Thermal light sources, such as diaphragm sources or spiral-wound filaments, lasers or light-emitting diodes (especially in the IR and UV ranges) are used as light sources. Thermal light sources emit in a very broad spectral band. However, this causes a lot of energy to flow into spectral ranges that are not used to analyze and calculate gas concentrations. Such gas sensors therefore have low efficiency in terms of their energy consumption. In addition, their output cannot be modified rapidly without additional mechanical components, which limits the use of noise suppression methods at a simultaneously fast response time.
By contrast, lasers lead mostly to a very good signal-to-noise ratio, but they are relatively expensive and must be thermally stabilized.
Light-emitting diodes (LEDs) are substantially more cost-effective than lasers and emit in a limited spectral range, so that they are more efficient in terms of their energy consumption than thermal light sources. In addition, just like lasers, LEDs lend themselves to fast electric modulation. On the other hand, the spectral emission characteristic of LEDs shows a thermal drift, which is noticeable in a disturbing manner in thermally non-stabilized optical gas sensors.